The ENSO is currently in the neutral range. The oceanic indices have responded better and favourably in recent weeks towards the likelihood of La Nina. The atmospheric index SOI, the oceanic linkages ONI and the trade winds have strengthened to push for La Nina, early next year. However, there is still a disparity between the CPC ENSO Outlook and IRI model-based ENSO forecast. While the former (issued on Dec 23, 2024) forecasts La Nina onset in Jan 2025, the objective IRI outlook does not promise a La Nina event and inclines towards the continuation of ENSO-neutral conditions for Dec-Feb 2025. Going further forward, the CPC ENSO outlook predicts a clear preference for La Nina during the winters of 2025, followed by a return to an ENSO-neutral state during the boreal spring of 2025.
ENSO: Equatorial sea surface temperatures are near to below average in the central and eastern Pacific Ocean. The strength of trade winds in the lower levels, which was earlier reduced in Nov 2024, has signs of picking up. However, the change in the SSTs, sub-surface temperatures and intensification of trade winds need to be sustained for a smooth transition from ENSO-Neutral to La Nina.
The Nino indices have all turned negative in the central and eastern Pacific Ocean. The dominating part is the Nino 3.4 region, the marker for ONI has taken big strides over the last 2 weeks. From its earlier mark of -0.3°C on 10 Dec 2024, the index has leapt to -0.8°C, crossing the threshold mark of -0.5°C for the second consecutive week. The average value of ONI for the month of Dec 2024 is -0.5°C, just breaching the threshold mark. Any further drop in the sea surface temperature will heighten the likelihood of La Nina, possibly in Jan 2025.
IOD: The Indian Ocean Dipole is neutral. The index had been tending negative from mid-Oct but returned to neutral value at the start of Dec 2024. This narrowly averted the negative IOD event and is now safe enough to remain neutral throughout the forecast period to April 2025. The latest value of the IOD index for the week ending 22 Dec 2024 was -0.26°C. This is in line with the typical IOD behaviour at this time of the year.
MJO: The Madden-Julian Oscillation is likely to propagate eastward from the West Pacific to the Western Hemisphere & Africa in Phase 8. The amplitude is expected to drop and become the least during the first week of Jan 2025. The growing La Nina response, coupled with negative neutral IOD will disrupt the MJO signal over the next few weeks. The Indian seas are likely to remain passive for the next two weeks. As such, the Northeast Monsoon is likely to withdraw anytime soon from the Indian region leaving Peninsular India with the least weather activity.
The far western position of MJO in the Pacific, and an ongoing negative-neutral IOD favour growing atmospheric response to La Nina conditions. The earliest emergence of La Nina could be in Jan 2025, subject to the continuation of the decline in the sea surface temperature in the Nino region. The stint of La Nina, at best, will be very short and sweet. Therefore, the winter season of 2025 may remain recluse to the emerging La Nina conditions and sail through without any major disruptions.